Encapsulated phase change cell structures and methods

ABSTRACT

Methods and devices associated with phase change cell structures are described herein. In one or more embodiments, a method of forming a phase change cell structure includes forming a substrate protrusion that includes a bottom electrode, forming a phase change material on the substrate protrusion, forming a conductive material on the phase change material, and removing a portion of the conductive material and a portion of the phase change material to form an encapsulated stack structure.

TECHNICAL FIELD

The present disclosure relates generally to semiconductor memory devicesand methods, and more particularly, to phase change cell structures andmethods.

BACKGROUND

Memory devices are typically provided as internal, semiconductor,integrated circuits in computers or other electronic devices. There aremany different types of memory including random-access memory (RAM),read only memory (ROM), dynamic random access memory (DRAM), synchronousdynamic random access memory (SDRAM), phase change random access memory(PCRAM), and flash memory, among other types of memory.

Resistance variable memory devices, such as PCRAM devices, can include astructural phase change material such as a chalcogenide alloy, forinstance, which can be programmed into different resistivity states tostore data. The phase change memory cells are non-volatile and theparticular data stored in a phase change memory cell can be read bysensing the cell's resistance, e.g., by sensing current and/or voltagevariations based on the resistance of the phase change material.

In cases in which the resistance variable memory device includes achalcogenide alloy, the chalcogenide alloy can exhibit a reversiblestructural phase change, e.g., from amorphous to crystalline. A smallvolume of the chalcogenide alloy can be integrated into a circuit thatcan allow the cell to act as a fast switching programmable resistor.This programmable resistor can exhibit greater than 40 times the dynamicrange of resistivity between the crystalline state (low resistivity) andthe amorphous state (high resistivity), and is also capable ofexhibiting multiple intermediate states that allow multi-bit storage ineach cell. That is, resistance variable memories may achieve multi-levelcell (MLC) functionality via programming of memory cells to one of anumber of different resistance levels.

Various previous processes used to form phase change memory cells canlead to contamination of the phase change material due to exposure ofthe phase change material to plasma, oxygen, and humidity, among othercontaminants. These contaminants can cause a reduction in cellperformance and cell failure. Also, previous formation processes canresult in problems such as delamination of the phase change memory cellsfrom the substrate, among other problems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a portion of a phase change memory array thatcan include phase change structures in accordance with one or moreembodiments of the present disclosure.

FIG. 2 illustrates an example of pulses that can be used to programphase change memory cells in accordance with one or more embodiments ofthe present disclosure.

FIG. 3 illustrates a cross-sectional view of a phase change cellstructure according to the prior art.

FIG. 4 illustrates a cross-sectional view of a phase change cellstructure in accordance with one or more embodiments of the presentdisclosure.

FIGS. 5A-5D are cross-sectional views illustrating formation of a phasechange cell structure in accordance with one or more embodiments of thepresent disclosure.

DETAILED DESCRIPTION

Methods, devices, and systems associated with phase change cellstructures are described herein. In one or more embodiments, a method offorming a phase change cell structure includes forming a substrateprotrusion that includes a bottom electrode, forming a phase changematerial on the substrate protrusion, forming a conductive material onthe phase change material, and removing a portion of the conductivematerial and a portion of the phase change material to form anencapsulated stack structure.

One or more embodiments of the present disclosure can provide variousbenefits, such as, reducing etch damage, contamination, and/ordelamination during the formation of the phase change cell structures,among other benefits. For instance, one or more embodiments can reducethe reset current requirement for the phase change cell structures andreduce thermal crosstalk associated with phase change memory cells,which can provide various benefits including improved data reliabilityand retention and increased read and/or write times, among various otherbenefits.

In the following detailed description of the present disclosure,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration how one or more embodimentsof the disclosure may be practiced. These embodiments are described insufficient detail to enable those of ordinary skill in the art topractice the embodiments of this disclosure, and it is to be understoodthat other embodiments may be utilized and that process, electrical,and/or structural changes may be made without departing from the scopeof the present disclosure.

The figures herein follow a numbering convention in which the firstdigit or digits correspond to the drawing figure number and theremaining digits identify an element or component in the drawing.Similar elements or components between different figures may beidentified by the use of similar digits. For example, 420 may referenceelement “20” in FIG. 4, and a similar element may be referenced as 520in FIG. 5A. As will be appreciated, elements shown in the variousembodiments herein can be added, exchanged, and/or eliminated so as toprovide a number of additional embodiments of the present disclosure. Inaddition, the proportion and the relative scale of the elements providedin the figures are intended to illustrate various embodiments of thepresent invention and are not to be used in a limiting sense.

As used in this disclosure, the terms “wafer” and “substrate” are usedinterchangeably and are to be understood as includingsilicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology,doped and undoped semiconductors, epitaxial layers of silicon supportedby a base semiconductor foundation, and other semiconductor structures.Furthermore, when reference is made to a “wafer” or “substrate” in thefollowing description, previous process steps may have been utilized toform regions or junctions in the base semiconductor structure orfoundation.

FIG. 1 is a schematic of a portion of a phase change memory array thatcan include phase change structures in accordance with one or moreembodiments of the present disclosure. In the embodiment illustrated inFIG. 1, the memory array 100 includes a number of phase change memorycells each having an associated access device 102 and resistancevariable element 104, e.g., a phase change cell structure 104. Theaccess devices 102 can be operated, e.g., turned on/off, to access thememory cells in order to perform operations such as data programming,e.g., writing, and/or data reading operations on the resistance variableelements 104.

In the embodiment illustrated in FIG. 1, the access devices 102 aremetal oxide semiconductor field effect transistors (MOSFETs). As shownin FIG. 1, a gate of each MOSFET 102 associated with each memory cell iscoupled to one of a number of access lines 105-0 (WL0), 105-1 (WL1), . .. , 105-N (WLN), i.e., each access line 105-0, 105-1, . . . , 105-N iscoupled to a row of phase change memory cells. The access lines 105-0,105-1, . . . , 105-N may be referred to herein as “word lines.” Thedesignator “N” is used to indicate that a memory array can include anumber of word lines. The resistance variable elements 104 can be aphase change cell structure having a phase change material that includeschalcogenide alloys such as a Germanium-Antimony-Tellurium (GST)material, e.g., a Ge—Sb—Te material such as Ge₂Sb₂Te₅, Ge₁Sb₂Te₄,Ge₁Sb₄Te₇, etc. The hyphenated chemical composition notation, as usedherein, indicates the elements included in a particular mixture orcompound, and is intended to represent all stoichiometries involving theindicated elements. Other phase change materials can include Ge—Te,In—Se, Sb—Te, Ga—Sb, In—Sb, As—Te, Al—Te, Ge—Sb—Te, Te—Ge—As, In—Sb—Te,Te—Sn—Se, Ge—Se—Ga, Bi—Se—Sb, Ga—Se—Te, Sn—Sb—Te, In—Sb—Ge, Te—Ge—Sb—S,Te—Ge—Sn—O, Te—Ge—Sn—Au, Pd—Te—Ge—Sn, In—Se—Ti—Co, Ge—Sb—Te—Pd,Ge—Sb—Te—Co, Sb—Te—Bi—Se, Ag—In—Sb—Te, Ge—Sb—Se—Te, Ge—Sn—Sb—Te,Ge—Te—Sn—Ni, Ge—Te—Sn—Pd, and Ge—Te—Sn—Pt, among various other phasechange materials.

In the embodiment illustrated in FIG. 1, each resistance variableelement 104 is coupled to one of a number of data lines 107-0 (BL0),107-1 (BL1), 1107-M (BLN), i.e., each data line 107-0, 107-1, . . . ,107-M is coupled to a column of phase change memory cells. The datalines 107-0, 107-1, . . . , 107-M may be referred to herein as “bitlines” or “sense lines.” The designator “M” is used to indicate that amemory array can include a number of bit lines. For ease of addressingin the digital environment, the number of word lines 105-1, . . . ,105-N and the number of bit lines 107-1, . . . , 107-M can each be somepower of two, e.g., 256 word lines by 4,096 bit lines. However,embodiments are not limited to particular numbers of word lines and/orbit lines.

In operation, appropriate voltage and/or current signals, e.g., pulses,can be applied to the bit lines 107-0, 107-1, . . . , 107-M and wordlines 105-0, 105-1, . . . , 105-N in order to program data to and/orread data from the phase change memory cells of the array 100. As anexample, the data stored by a phase change memory cell of array 100 canbe determined by turning on an access device, e.g., 102, and sensing acurrent passing through the phase change cell structure, e.g., 104. Thecurrent sensed on the bit line associated with the memory cell beingread, e.g., bit line 107-0, 107-1, . . . , 107-M, corresponds to aresistance level of the phase change cell structure 104, which in turncorresponds to a particular data value, e.g., a binary value such as 1,0, 001, 111, 0101, etc.

Embodiments of the present disclosure are not limited to the examplearray 100 illustrated in FIG. 1. For example, as one of ordinary skillin the art will appreciate, the access device 102 associated with aparticular memory cell can be a device other than a MOSFET. In someembodiments, the access device 102 can be a bipolar junction transistor(BJT) or a diode, among other types of access devices. Also, a memoryarray, e.g., 100, can have an architecture other than that illustratedin FIG. 1, as will be understood by one of ordinary skill in the art.

In one or more embodiments, the access device associated with the phasechange memory cells in an array can be a diode. The diode can be anumber of types of diodes, such as a p-n diode, a Zener diode, or aSchottky diode, among various other types of diodes.

In operation, appropriate voltage and/or current signals, e.g., pulses,can be applied to the bit lines and word lines in order to program datato and/or read data from the phase change memory cells of the array. Asan example, the data stored by a phase change memory cell of array canbe determined by turning on a diode access device and sensing a currentpassing through the phase change element. The current sensed on the bitline associated with the memory cell being read corresponds to aresistance level of the phase change element, which in turn correspondsto a particular data value, e.g., a binary value such as 1, 0, 001, 11,1011, etc.

As one of ordinary skill in the art will appreciate, the phase changememory array 100 illustrated in FIG. 1 can be coupled to programming,e.g., write, circuitry and/or sensing, e.g., read, circuitry (not shownin FIG. 1). For instance, the array 100 can be coupled to write and/orread circuitry as described below in connection with FIG. 6.

FIG. 2 illustrates an example of pulses that can be used to programphase change memory cells in accordance with one or more embodiments ofthe present disclosure. In FIG. 2, the pulse 211 represents anamorphizing (reset) pulse, e.g., a pulse used to place one or more phasechange memory cells in an amorphous (high resistivity) state. The pulse213 represents a crystallizing (set) pulse, e.g., a pulse used to placeone or more phase change memory cells in a crystalline (low resistivity)state. The reset pulse 211 and the set pulse 213 can be applied to aparticular memory cell in order to alter the resistance of the phasechange cell structure, e.g., phase change cell structure 104 shown inFIG. 1, by raising/lowering the temperature of the phase change materialcorresponding to the cell in a manner such that the resistance of thecell is changed, e.g., programmed, to a value that corresponds to aparticular desired data state.

As one of ordinary skill in the art will appreciate, a reset pulse suchas reset pulse 211 can be used to place the phase change material, e.g.,phase change cell structure 104 shown in FIG. 1, or a portion thereof,in a relatively amorphous state corresponding to a relatively highresistance value, e.g., about 100 kiloohm to 1 megaohm. For instance, inthe example illustrated in FIG. 1, the reset pulse 211 can be used toraise the temperature of the phase change material to a temperature Tasufficient to melt the phase change material; the phase change materialcools over a short time period, i.e., t1, to amorphize the phase changematerial such that the phase change material does not re-form someportion of its internal crystalline structure. The time t1 can bereferred to as a “quenching time.”

A set pulse, such as set pulse 213 illustrated in FIG. 2, can be used toraise the temperature of a phase change material above a temperature Txand maintain the temperature of the phase change material for a time,e.g., t2, sufficient to allow crystallization of the phase changematerial to occur. As such, the set pulse 213 can place the phase changematerial in a relatively crystalline state corresponding to a relativelylow resistance value, e.g., about 1 kiloohm to 10 kiloohm, for instance.

Embodiments of the present disclosure are not limited to the resetand/or set pulses illustrated in the example shown in FIG. 2. As anexample, one or more embodiments of the present disclosure can providean electrode encapsulated phase change cell structure which laterallydistributes the electric field in the phase change material portion anddissipates heat through the sides that can shorten the quench time,e.g., t1 shown in FIG. 2, associated with a reset pulse, e.g., 211. Forinstance, various embodiments can increase the quench rate associatedwith a reset pulse by laterally dissipating heat and improve the resetprocess by laterally distributing current to facilitate the formation ofhemispherical amorphous regions that covers the bottom electrode, whichcan reduce immature reset. A lateral current distribution also canreduce heat loss through the top electrode and heat-induced topelectrode delamination. The encapsulating top electrode can improvethermal isolation of a phase change cell structure from adjacentstructures to reduce thermal crosstalk. As one example, in someembodiments, the time for a reset operation can be about 10 ns.

FIG. 3 illustrates a cross-sectional view of a phase change cellstructure according to the prior art. The phase change cell structure inFIG. 3 illustrates a structure according to prior art. In FIG. 3, twophase change cell structures 304-1 and 304-2 are formed on metalcontacts 322-1 and 322-2 in a dielectric layer 320. The phase changecell structures 304-1 and 304-2 include bottom electrodes 326-1 and326-2 formed in the substrate 324. The substrate 324 can be siliconnitride (SiN) silicon oxide (SiO), among other suitable substratematerials.

In FIG. 3, the phase change material portions 330-1 and 330-2, whichinclude active regions 332-1 and 332-2, and the top electrodes 328-1 and328-2 are formed over the substrate 324. The individual phase changecell structures 304-1 and 304-2 are formed by masking a portion of thetop electrode material and etching away a portion of the top electrodematerial and the phase change cell material. The etching of the topelectrode material and the phase change cell material to form the phasechange cell structures 304-1 and 304-2 exposes the sidewalls of thephase change cell structures 304-1 and 304-2 to ambient oxidation andhumidity, along with the etchant. This exposure can contaminate thephase change cell structures causing oxidation, top electrode/phasechange material portion interface damage, and introduce contaminates inthe phase change material, such as spikes in titanium (Ti). Thesecontaminants can cause degradation of device performance or failure ofthe device associated with the phase change cell structure.

In addition, the etching of the top electrode material and the phasechange cell material to form the phase change cell structure can causedelamination of the phase change material portion from the substrateduring the etch cleaning process. The delamination can be caused by theweak adhesion of the phase change material to the substrate under theforce exerted by a wet etch solution.

FIG. 4 illustrates a cross-sectional view of phase change cellstructures 404-1 and 404-2 in accordance with one or more embodiments ofthe present disclosure. The phase change cell structures 404-1 and 404-2in FIG. 4 illustrate a phase change cell stack that includes substrateprotrusions 425-1 and 425-2 encapsulated by phase change materialportions 430-1 and 430-2 and phase change material portions 430-1 and430-2 at least partially encapsulated by top electrodes 428-1 and 428-2.In one or more embodiments, the phase change cell structure 404-1 can bepart of a first line stripe configuration and phase change cellstructure 404-2 can be part of a second line stripe configuration,wherein the first and second line strip configurations include a numberof phase cell structures. In one or more embodiments, the phase changecell structure 404-1 and phase change cell structure 404-2 can each bepart of a mesa configuration, wherein each mesa includes a phase changecell structure.

In FIG. 4, the phase change cell structures 404-1 and 404-2 includebottom electrodes 426-1 and 426-2 that are formed in the substrateprotrusions 425-1 and 425-2. The bottom electrodes 426-1 and 426-2 arecoupled to metal contacts 422-1 and 422-2. The metal contacts 422-1 and422-5 are formed in a dielectric 420. The phase change cell structures404-1 and 404-2 are coupled to other elements, such as transistors,diodes, and/or bit lines, among other elements, of a phase change memoryarray, such as the array illustrated in FIG. 1, via the metal contacts422-1 and 422-2 and/or the top electrodes 428-1 and 428-2.

In one or more embodiments, the bottom electrodes can have a diameter ofapproximately 50 nanometers (nm), among other dimensions. The phasechange material portion can have a diameter of approximately 100 nm anda depth of approximately 100 nm. Embodiments of the present disclosureare not limited to these examples.

In one or more embodiments, a phase change cell structure, e.g., 404-1,is isolated from another phase change cell structure, e.g., 404-2, byremoving a portion of the top electrode material at least partiallyencapsulating the phase change material and a portion of the phasechange material encapsulating the substrate protrusions to expose thesubstrate. The removal of a portion of the top electrode material andthe phase change material can be done by etching. When etching the topelectrode material and the phase change material, which encapsulates thephase change material portion and the substrate protrusion 425-1 and425-2, respectively, the contamination due to plasma, oxidation, andhumidity, described in association with the structure illustrated inFIG. 3, is limited to the recess region 431 of the phase change cellstructure away from the active switching region of the phase changematerial portion in FIG. 4. Also, the encapsulated substrate protrusionresults in more surface area for the phase change material to adhere tothe substrate and substrate protrusion. The electrode encapsulated phasechange material portion results in a phase change cell structure that isless susceptible to delamination during fabrication processes, such asthe etch clean process.

In various embodiments, the phase change material portion encapsulatedby the top electrode can laterally distribute the electric field in thephase change material portion. The laterally distributed electric fieldcan reduce the reset current for the phase change material portion andcan also reduce thermal cross talk between adjacent phase change memorycells. In various embodiments, the lateral distribution of the electricfield caused by the encapsulated phase change material portion canfacilitate the formation of a hemispherical amorphous region, e.g. 432-1and 432-2 in FIG. 4, that covers the upper surface, e.g., 423-1 and423-2 in FIG. 4, of the bottom electrode. A hemispherical amorphousregion that covers the bottom electrode can reduce and/or preventunwanted and/or premature reset of the phase change material. Thelateral electric field distribution can also reduce heat loss throughthe top electrode and reduce heat-induced top electrode delamination.The top electrode encapsulating the phase change material portion canalso thermally isolate the phase change material more efficiently toreduce thermal cross talk between adjacent phase change memory cells.

FIGS. 5A-5D are cross-sectional views illustrating formation of phasechange cell structures 504-1 and 504-2 in accordance with one or moreembodiments of the present disclosure. The phase change cell structures504-1 and 504-2 described in FIGS. 5A-5D includes a portion of a numberof phase change cell structures at various stages in a fabricationsequence.

FIG. 5A shows two phase change cell structures at a particular stage ina phase change memory device fabrication sequence. The embodimentillustrated in FIG. 5A includes a dielectric 520 with metal contacts522-1 and 522-2, which phase change cell structures 504-1 and 504-2illustrated in FIG. 5D are fabricated upon. The metal contacts 522-1 and522-2 can be formed by forming an opening in the dielectric 520 andfilling the opening with a conductor, such as metals and/or metal alloysincluding titanium (Ti), tungsten (W), or tantalum (Ta), among otherconductors. The metal and/or metal alloy can be planarized usingchemical mechanical planarization (CMP) or other suitable planarizationtechniques to create a planar surface that includes the top surface ofthe dielectric 520 and the metal contacts 522-1 and 522-2. The metalcontacts 522-1 and 522-2 can provide an electrical and thermalconductive path to couple the phase change cell structures to otherelements in the phase change memory cell, such as a transistor, diode,and/or a bit line, among other elements.

In FIG. 5A, a substrate 524 can be formed on the dielectric 520 andmetal contacts 522-1 and 522-2. An opening can be formed in thesubstrate 524 by removing a portion of the substrate 524 to the metalcontacts 522-1 and 522-2 via etching. The bottom electrodes 526-1 and526-2 can be formed by filling the openings in the substrate with thebottom electrode material.

In FIG. 5B, substrate protrusions 525-1 and 525-2 are formed. A portionof the substrate 524 is removed to form the substrate protrusions 525-1and 525-2. In various embodiments, the substrate 524, e.g., as shown inFIG. 5A, is photo masked to protect the substrate protrusion and thesubstrate is etched to a certain depth leaving a portion of thesubstrate 524 that forms the substrate protrusions 525-1 and 525-2.Substrate protrusions 525-1 and 525-2 can provide the base for theformation of the encapsulated phase change material portion, e.g., asshown in FIG. 5C.

FIG. 5C shows two phase change cell structures at a particular stage ina phase change memory device fabrication sequence. In FIG. 5C, a phasechange material 529 is formed over the substrate protrusions 525-1 and525-2 to encapsulate the substrate protrusions 525-1 and 525-2. Thesubstrate protrusions 525-1 and 525-2 can include sidewalls that can becovered by the phase change material 529. A top electrode 527 is formedover the phase change material 529. The phase change material 529 caninclude sidewalls that are covered by the top electrode 527. The topelectrode 527 can encapsulate at least the active region of the phasechange material portion, e.g., phase change material portions 530-1 and530-2 shown in FIG. 5D. The phase change material 529 and the topelectrode 527 can be formed using a conformal deposition process, suchas atomic layer deposition (ALD) and/or a non-conformal depositionprocess, such as physical vapor deposition (PVD), among other processes.The formation of the phase change material and the top electrode overthe substrate protrusions 525-1 and 525-2 can create a recess 531between the phase change cell structures. The recess 531 can be used toexecute further process steps on the phase change cell structures thatare away from the active region of the phase change material portion.For example, further etching and cleaning of the phase change cellstructures after formation of the recess will not harm the activeportion of the phase change cell structure.

FIG. 5D shows two phase change structures at a particular stage in aphase change memory device fabrication sequence. In FIG. 5D, phasechange cell structures 504-1 and 504-2 are formed and isolated from eachother. Phase change material portion 530-1 and top electrode 528-1 areisolated from phase change material portion 530-2 and top electrode528-2 by removing a portion of the phase change material 529 and the topelectrode 527 in the recess 531 between the substrate protrusions 525-1and 525-2. In one or more embodiments, the portion of the phase changematerial 529 and the top electrode 527 can be removed by etching. In oneor more embodiments, a portion of the phase change material 529 and thetop electrode 527 can be photo patterned and then a portion of the phasechange material 529 and the top electrode 527 can be removed by etching.The removal of a portion of the phase change material 529 and the topelectrode 527 can isolate the phase change cell structures 504-1 and504-2 by exposing a portion of the substrate 524.

Methods and devices associated with phase change cell structures aredescribed herein. In one or more embodiments, a method of forming aphase change cell structure includes forming a substrate protrusion thatincludes a bottom electrode, forming a phase change material on thesubstrate protrusion, forming a conductive material on the phase changematerial, and removing a portion of the conductive material and aportion of the phase change material to form an encapsulated stackstructure.

It will be understood that when an element is referred to as being “on,”“connected to” or “coupled with” another element, it can be directly on,connected, or coupled with the other element or intervening elements maybe present. In contrast, when an element is referred to as being“directly on,” “directly connected to” or “directly coupled with”another element, there are no intervening elements or layers present. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements and that these elementsshould not be limited by these terms. These terms are only used todistinguish one element from another element. Thus, a first elementcould be termed a second element without departing from the teachings ofthe present disclosure.

Although specific embodiments have been illustrated and describedherein, those of ordinary skill in the art will appreciate that anarrangement calculated to achieve the same results can be substitutedfor the specific embodiments shown. This disclosure is intended to coveradaptations or variations of various embodiments of the presentdisclosure.

It is to be understood that the above description has been made in anillustrative fashion, and not a restrictive one. Combination of theabove embodiments, and other embodiments not specifically describedherein will be apparent to those of skill in the art upon reviewing theabove description. The scope of the various embodiments of the presentdisclosure includes other applications in which the above structures andmethods are used. Therefore, the scope of various embodiments of thepresent disclosure should be determined with reference to the appendedclaims, along with the full range of equivalents to which such claimsare entitled.

In the foregoing Detailed Description, various features are groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the disclosed embodiments of the presentdisclosure have to use more features than are expressly recited in eachclaim.

Rather, as the following claims reflect, inventive subject matter liesin less than all features of a single disclosed embodiment. Thus, thefollowing claims are hereby incorporated into the Detailed Description,with each claim standing on its own as a separate embodiment.

1. A method of forming a phase change cell structure, the methodcomprising: forming a bottom electrode in a substrate material; forminga substrate protrusion that includes the bottom electrode by removingportions of the substrate material while maintaining encapsulation ofthe bottom electrode with the substrate material; forming a phase changematerial on the substrate protrusion; forming a conductive material onthe phase change material; and removing a portion of the conductivematerial and a portion of the phase change material to form anencapsulated stack structure, wherein removing the portion of theconductive material and the portion of the phase change materialisolates the encapsulated stack structure from an adjacent encapsulatedstack structure.
 2. The method of claim 1, wherein the method includesforming the substrate protrusion by photo patterning a substrate andetching the substrate.
 3. The method of claim 1, including forming thephase change material using a non-conformal deposition method.
 4. Themethod of claim 1, including forming the conductive material using anon-conformal deposition method.
 5. A method of forming a phase changememory structure, the method comprising: forming a first phase changecell stack on a substrate, wherein the first phase change cell stackincludes a phase change material portion that encapsulates a firstsubstrate protrusion and a top electrode formed on the phase changematerial portion; forming a second phase change cell stack on thesubstrate, wherein the second phase change cell stack includes the phasechange material portion that encapsulates a second substrate protrusionand the top electrode formed on the phase change material portion; andisolating the first phase change cell stack from the second phase changecell stack by removing a portion of the top electrode and a portion ofthe phase change material portion; and wherein forming the first phasechange cell stack includes: forming a first bottom electrode in asubstrate material; and forming the first substrate protrusion byremoving portions of the substrate material while maintainingencapsulation of the first bottom electrode with the substrate material.6. The method of claim 5, including forming the top electrode such thatthe top electrode encapsulates the phase change material portion.
 7. Themethod of claim 5, including etching the phase change material portionand the top electrode to expose the substrate.
 8. The method of claim 5,wherein the method includes forming the first phase change cell stack byforming the first bottom electrode in the first substrate protrusion andforming the second phase change cell stack by forming a second bottomelectrode in the second substrate protrusion.
 9. The method of claim 8,including forming the first phase change cell stack on a first metalcontact and forming the second phase change cell stack on a second metalcontact.
 10. The method of claim 9, including coupling the first bottomelectrode to the first metal contact and coupling the second bottomelectrode to a second metal contact.